It is known to produce a cross-oriented multi-layer plastic film by a blown film extrusion process in which a tubular plastic film with outer and inner plastic film layers is co-extruded from concentric annular die orifices to form a bubble which, during its travel to a collapsing station, is twisted by rotation about the direction of travel of the bubble between the extruding station and the collapsing station. The plastic film layers are extruded with molecular orientation in the direction of travel of the bubble, and the twisting of the bubble causes the molecular orientation to become inclined to the direction of travel of the bubble so that collapsing of the bubble produces a cross-oriented four-layer plastic film. Such a process is disclosed for example in U.S. Pat. No. 4,358,330 (Aronovici) issued Nov. 9, 1982, see column 6 lines 4 to 32.
According to the teaching in the Aronovici patent, the die is rotated at a certain speed which, in combination with the take off speed of the rotating cylinders at the collapsing station and the blow ratio, acts to relocate the frost line of the tubular film so that the desired molecular orientation is imparted to the film layers prior to the solidification of the extruded viscous molten plastic material. According to Aronovici, the die should be rotated at a speed in the range of between about 1 and 20 rpm, i.e. a speed substantially higher than the previously known conventional speed range of between 0.1 and 0.5 rpm. However, such higher die rotation speed and relocation of the frost line is liable to produce bubble instability which adversely affects cross-orientation. So far as applicant is aware, the Aronovici teaching has not been commercially successful.
It is therefore an object of the invention to provide an improved process for producing cross-oriented multi-layer plastic film.